It is a common practice to use more than one local oscillator to tune the full frequency range of television RF sources, which for a television receiver in the United States, can extend from 55 MHz of channel 2 of the lower VHF band to 847 MHz of channel 69 of the UHF band, a frequency range of more than 17:1. This requires that the local oscillators for conventional all band TV tuners cover a wide frequency range of from approximately 90 MHz up to almost 1 GHz.
It is desirable that a local oscillator not be required to tune over more than a range of 2:1, with an outside limit of about 3:1, due to problems of assuring that the oscillator will oscillate, reliability of frequency and signal amplitude of the generated oscillator signal over the wide spectrum, and tracking problems, since other circuits, e.g., tunable filters, must track as the local oscillator frequency is changed. Further, varactor diodes, which typically are used to tune the local oscillator, have a limited range of adjustment and become non-linear in the low capacitance part of their adjustment range. For these reasons, in a television receiver, at a minimum, two local oscillators are often used with each covering only part of the tunable spectrum, e.g., one for the VHF band and the other for the UHF band.
It is further desirable that a single oscillator be used in order to eliminate beat frequency problems between oscillators, which requires that oscillators not in use be turned off. Still further, a reduced number of oscillators would result in reduced components, reduced printed circuit space, and reduced cost.
A known apparatus which reduces the local oscillator tuning range is described by H. Fuchs in DE 38 30 921 entitled ARRANGEMENT FOR ADAPTING A RECEIVER TO DIFFERENT CHANNEL RASTERS. Fuchs describes apparatus for generating a conversion signal for a mixer which includes a variable oscillator for generating a first RF signal over a predetermined frequency range, a divider for converting the frequency of the first RF signal in accordance with a binary number, N, to provide a second RF signal having a frequency range which varies as a function of the binary number, N, for application to the mixer; and a microprocessor for selecting values of the binary number, N, to provide a respective frequency range for the second RF signal for each selected value of the binary number, N, for the purpose of reducing memory requirements in a multi-stantard television receiver.
In certain applications, such as applications requiring quadrature demodulation, it would be desirable to provide a quadrature phase shifted output signal for a second mixer. An example of a digital quadrature phase shift generator is described by Huber et al. in U.S. Pat. No. 5121057 entitled MEDIA FLAW DETECTION APPARATUS FOR A MAGNETIC DISC DRIVE WITH SQUARING AND SUMMING OF IN-PHASE AND QUADRATURE-PHASE DETECTED SIGNALS. The Huber et al. apparatus includes a divide-by- four circuit for generating in-phase and quadrature signals for application to respective mixers.